Characterisation of the adsorbate-adsorbent interaction between drugs or pesticides and carbon/silica compounds

Characterisation of the Adsorbate-

Adsorbent interaction between

Drugs or Pesticides and

CarbonlSilica Compounds

Charlotte May

van

Eeden

MSc.

(Pharm)

Thesis submitted for the degree Philosophiae Doctor in

Pharmaceutics at the North-West University, Potchefstroom

Campus

Promoter

:

Prof. Melgardt

M.

de Villiers

Co-Promoter

:

Prof. Wilna Liebenberg

Potchefstroom

2005

TABLE OF CONTENTS

TABLE OF CONTENTS

ABSTRACT

UITTREKSEL

AIMS

AND

OBJECTIVES

1

vii ix

xi

PART I

-

Characteristics and Mechanisms of Environmentally and

Biologically Important Adsorbate-Adsorbent Interactions

1

CHAPTER

1

-

Fundamentals of Adsorption and Adsorption Processes

3 Introduction

Important thermodynamic properties regulating adsorption Importance of fundamental solution theory for adsorption Partition between two separate phases

Fundamentals of the adsorption theory

Langmuir adsorption isotherm Freundlich equation

Brunauer-Emmet-Teller (BET) adsorption theory Polanyi adsorption potential theory

Surface properties and areas of solids Isosteric heat of adsorption

Adsorbent partition and concentration Conclusion

CHAPTER 2

-

Adsorption of Organic Substances in Environmental and

Biological Systems

22

Introduction 22

Adsorption to soil and other natural adsorbents 23

Influence of s o ~ t i o n on adsorbate activity 26

Contaminant uptake by plants from soil and water 27 Pharmaceutically important adsorption at the solid-liquid interface 27

A d s o ~ t i o n of toxic substances from the gastrointestinal tract 28

Adsorption problems in drug formulation 28

Influence of adsorption on the bioavailability of drugs 29

Conclusion 32

PART

II

-

Effect of Changes in the Physicochemical Properties of the

Insecticide Amitraz on its Chemical Fate in the Environment in the Absence

and Presence of Important Adsorbate-Adsorbent Interactions

33

CHAPTER

3

-

Physicochemical, Biological and Toxicological Properties of the

Formamidine Insecticide Amitraz

35

Introduction

Toxicological effects Ecological toxicity

Stability, environmental fate and metabolism of amitraz Physicochemical properties

CHAPTER

4

-

Solvent and Surfactant Enhanced Solubilisation, Stabilisation

and Degradation of Amitraz

43

Introduction 43

Materials and Methods Materials

HPLC and W-spectroscopic analysis of amitraz

Solubility of amitraz in surfactant and co-solvent solutions Stability of amitraz as afunction ofpH

Stability of amitraz in organic solvents 46

Stability of amitraz in surjbctant solutions 46

Mass spectrometric identification of amitraz degradation products 47

Results and Discussion 47

Dissolution of amitraz in surfnctant and co-solvent solutions 49

Effect of changes in buffer composition, concentration, and ionic strength on amitraz

hydrolysis 50

pH rate profile for amitraz hydrolysis 54

Effect of an increase in temperature on amitraz hydrolysis Hydrolysis of amitraz in aqueous organic solvents

Hydrolysis of amitraz in aqueous sufactant solutions Conclusion

CHAPTER

5

-

The

Effect of Solubilising Agents on the Stability of Amitraz

Adsorbed onto Silica and Carbon Substrates

65

Introduction 65

Material and methods 66

Materials 66

Morphology of the adsorbent surfnces 68

HPLC analysis 69

Sample extraction 70

Amitraz sorption to adsorbents 7 1

Degradation kinetics of sorbed amitraz 71

Amitraz sorption on fruit and its removal 7 1

Results and discussion 72

Adsorption of amitraz to adsorbents 73

Capacity of the absorbents to adsorb amitraz 80

Adsorption of amitraz to fruit 85

Kinetics of the hydrolysis of sorbed amitraz 87

Conclusions 92

CHAPTER

6

-

Structural Characterisation, Physicochemical Properties,

Suspension Stability and Adsorption Properties of Four Crystal Forms of

Amitraz

93

Introduction

Material and methods Materials

Thermal analysis

Preparation of crystal forms Morphology of crystal forms X-ray powder dzflraction

Infrared spectroscopy HPLC analysis

Solubility and intrinsic dissolution measurements Degradation kinetics of sorbed and suspended amitraz Results and discussion

Preparation of crystal forms Morphology of the crystal forms

Solubility and dissolution properties of the crystal forms Crystal structures of the crystal forms

Stability of the crystal forms Conclusions

PART I11

-

The Effect of OTC Products Containing Known Adsorbents on the

Enterosorption of Drugs from the Gastrointestinal Tract

121

CHAPTER 7

-

In-Vitro evaluation of the Effect of OTC Products Containing

Activated Charcoal or Chitosan on the Enterosorption

of

Drugs in the Gastro-

Intestinal Tract

127

Introduction

Material and methods Materials

In-vitro adsorption studies HPLC analysis

Results and discussion 134

Acetaminophen adsorption Fluonetine HCI adsorption

Cimetidine adsorption Prazosin HCl adsorption

Conjugated estrogen adsorption

Conclusion

CHAPTER 8

-

Conclusion

REFERENCES

Characterisation of the Adsorbate-Adsorbent Interaction

between Drugs or Pesticides and CarbonISilica Compounds

Abstract

Administering adsorbates such as activated charcoal can treat acute poisoning from chemicals and pesticides. It has been suggested that activated charcoal is an effective antidote for virtually all organic and inorganic compounds.

The aim of this study was to characterise the adsorbate-adsorbent interactions. Adsorbents used were chitosan, activated charcoal, silica and humic acids. Adsorbates used were paracetamol, prazosin hydrochloride, cimetidine, fluoxetine hydrochloride, conjugated estrogens and amitraz. Amitraz is widely used in South Africa and amitraz adsorption studies were performed to gain an insight into effective pesticide waste and dip vat management.

This study used different solution properties to determine their influence on the hydrolysis of amitraz. Amitraz hydrolysis could be described as a pseudo-first order rate process and a type ABCD pH rate profile. Hydrolysis increased with temperature and was fastest at low pH, slowest at neutral to slightly alkaline pH, and slightly increased above pH 10. Hydrolysis was fastest in water, slower in propylene glycol and ethanol solutions and slowest in DMSO mixtures. In surfactant solutions, anionic micelles enhanced and cationic micelles retarded the hydrolysis rate. The half-live of amitraz was reduced from 27 days for the aqueous suspension in buffer pH 5.8 containing 0.5 % sodium lauryl sulphate to 8 hours and 12 hours when 1 % potassium oxihumate was added.

Adsorbents were mixed with a m i k solution for 24 hours at 31°C. Study results proved that coarse activated charcoal powder adsorbed more than the other adsorbents used and can be used to treat amitraz poisoning or to manage spills. A study was also done to investigate amitraz adsorption on pears and oranges. Fruit soaked in amitraz solution for 5 minutes and left to dry for 24 hours, were washed in solutions of distilled water, sodium lauryl sulphate, cetrimide and Tween 80.

Four crystal forms of amitraz were identified by their crystal morphology, XRPD patterns, aqueous solubility and thermal properties. Form C was the most stable with t% of 136 days. Forms B and D were least stable with tK of 28 days. Stability

correlated with solubility differences. The addition of sodium lauryl sulphate increased hydrolysis (t% = 17 hours) and no difference in stability of crystal forms in anionic surfactant solutions occurred.

Adsorption activity of activated charcoal and chitosan were done on OTC drugs. The official dissolution media, simulated gastric and intestinal fluids were used. Cimetidine did not adsorb onto activated charcoal or chitosan tablets. Adsorption of paracetamol was minimal. Prazosin hydrochloride and fluoxetine hydrochloride were strongly adsorbed by activated charcoal with no adsorption onto chitosan. Conjugated estrogens were adsorbed by chitosan and not by activated charcoal.

Anionic surfactants such as sodium lauryl sulphate can potentially be used for cleaning up amitraz, as it demonstrated increased solubilisation and hydrolysis of amitraz. Activated charcoal can be used to treat many drug poisonings and overdoses.

. . .

Karakterisering van die interaksie tussen koolstof/silikon-

verbindings as adsorbeermiddels en geneesmiddels of

pestisiede as geadsorbeerde stowwe

Uittreksel

Adsorbeermiddels soos geaktiveerde houtskool kan gebruik word om akute vergiftiging deur chemikaliee en pestisiede te behandel. Dit is voorgestel dat geaktiveerde koolstof 'n effektiewe teenmiddel vir bykans alle organiese en anorganiese verbindings is.

Die doe1 van hierdie studie was om die interaksie tussen adsorbeermiddel en geadsorbeerde stof te karakteriseer. Chitosaan, geaktiveerde koolstof, silika en humussure is as adsorbeermiddels gebruik en parasetamol, prasosienhidrochloried, simetidien, fluoksitienhidrochloried, gekonjugeerde estrogene en amitras as geadsorbeerde stowwe. Amitras word wyd in Suid-Afrika gebruik en adsorpsie- studies is gedoen om meer insig te verkry oor die effektiewe beheer van stortings van pestisiede en van dipstowwe.

In hierdie studie is verskillende oplosbaarheidseienskappe gebruik om die invloed daarvan op die hidrolise van amitras te bepaal. Die hidrolise van amitras is beskryf as a pseudo-eerste-orde proses en 'n tipe ABCD pH-snelheidprofiel. Hidrolise neem toe met 'n verhoging in temperatuur en was die vinnigste in die lae pH-gebied, stadigste by neutrale tot effens alkaliese pH en neem weer effens toe by pH bo 10. Hidrolise was die vinnigste in water, stadiger in propileenglikool en etanol en die stadigste in mengsels met DMSO. Anioniese miselle verhoog en kationiese miselle verlaag die snelheid van hidrolise. Die halfleeftyd van amitras neem af van 27 dae in 'n waterige suspensie wat met 0.5 % natruimlaurielsulfaat by pH 5.8 gebuffer is tot 8 uur en tot 12 uur as 1 % kaliumoksihumaat bygevoeg word.

Adsorbeermiddels is vir 24 uur by 3 1 OC met 'n oplossing van amitras gemeng. Die studie het getoon dat growwe houtskoolpoeier meer amitras adsorbeer as die ander adsorbeermiddels wat gebruik is en dit kan gebmik word om vergiftiging dew en stortings van amitras te behandel. Die adsorpsie van amitras op pere en lemoene is ook bestudeer. Die vrugte is vir 5 minute in 'n oplossing van amitras gedompel en vir

24 uur laat staan om droog te word. Daama is dit in oplossings van gedistilleerde water, natruimlaurielsulfaat, simetidien en Tween 80 gewas.

Vier kristalvorms van amitras is ge'identifiseer deur hul kristalmorfologie, XRPD- patrone, wateroplosbaarheid en termiese eienskappe. Vorm C was die stabielste met 'n t% van 136 dae. Vorms B en D was die onstabielste met 'n t% van 28 dae. Stabiliteit het gekorreleer met verskille in oplosbaarheid. Die byvoeging van natruim- laurielsulfaat verhoog hidrolise (t% = 17 we), maar geen verskil in die stabiliteit van

die kristalvorms is na byvoeging van oplossings van anioniese surfaktante gevind nie. Die eienskappe van adsorpsie van ODT-geneesmiddels op geaktiveerde koolstof en chitosaan is bestudeer. Die amptelike dissolusiemedia, sowel as gesimuleerde gastriese en intestinale vloeistowwe, is gebmik. Simetidien het nie aan geaktiveerde koolstof of chitosaantablette geadsorbeer nie. Die adsorpsie van parasetamol was minimaal. Prasosienhidrochloried en fluoksitienhidrochloried is sterk deur geaktiveerde koolstof geadsorbeer, maar het nie aan chitosaan geadsorbeer nie. Gekonjugeerde estrogene weer het aan chitosaan geadsorbeer, maar nie aan geaktiveerde koolstof nie.

Anioniese surfaktante soos natruimlaurielsulfaat kan moontlik gebruik word om stortings van amitras op te ruim aangesien dit solubilisasie en hidrolise verhoog. Geaktiveerde koolstof kan gebruik word om vergifiiging deur of oordosisse van verskeie geneesmiddels te behandel.

Characterisation of the Adsorbate-Adsorbent Interaction

between Drugs or Pesticides and CarbonISilica Compounds

Aims and Objectives of the Study

The aim of this study is to characterise the adsorbate-adsorbent interaction. The adsorbents that will be used in this study include chitosan, activated charcoal, silica and humic acids. Adsorbates that will be used include several drugs such as paracetamol, prazosin HCl, cimetidine, fluoxetine HC1 and conjugated estrogens. The pesticide that will be used is amitraz. The aim is to determine which adsorbate will form the best interaction to develop longer lasting drug release system or to be more useful in human poisoning treatment. The interaction between a dietary supplement and chronically used drugs will be studied.

Amitraz is a formamidine acaricide and insecticide effective against a wide variety of phytophagous mites and insects. Amitraz has moderate mammalian toxicity, is acutely toxic to fish and may affect avian reproduction. Amitraz is widely used in South Africa to control ticks in mobile and stationary spray and dip vats of up to 1000 L. Amitraz is also widely used as an acaricide against mites on fruit trees like pears, apples and citrus fruits. Through this process, large quantities of semiconcentrated pesticide waste are generated. To effectively manage these wastes, it is necessary to understand arnitraz better. Therefore, amitraz adsorption studies will be performed. Different crystal forms will be developed and characterised.

To achieve the aim of this study, several objectives must be met. These objectives are:

To investigate the characteristics and mechanisms of environmentally and biologically important adsorbate-adsorbent interactions. An indebt literature study on the fundamental of adsorption and adsorption processes will be done.

The effect of changes in the physicochemical properties of the insecticide amitraz on its chemical fate in the environment in the absence and presence of adsorbate- adsorbent interactions will be studied.

The physicochemical, biological and toxicological properties of amitraz will be studied by doing a literature study.

The solubilisation, stabilisation and degradation of amitraz in deferent solvents will be studied. Solvents that will be used are buffer solutions and surfactant solutions. A stability profile will be developed for amitraz.

The effect of solubilising agents on the stability of amitraz adsorbed onto silica and carbon substrates will be studied. Through this study the best adsorbent for amitraz will be determined.

A study on the adsorption of amitraz onto different fruits will be done. These h i t s include pears and oranges. The effect of different solutions on this adsorption will also be studied. Thereby the best method to wash fiuits sprayed with amitraz, can be determined.

Different crystal forms of amitraz will be studied for structural characterisation, physicochemical properties, suspension stability and adsorption properties. This will be performed by using TGA, DSC, infia red, X-ray, electron microscope and HPLC methods.

The in vitro adsorption effect of over the counter known adsorbents on drugs will be studied. Through this study the interaction can be studied and the best treatment can be suggested. Also the interaction between adsorbent and chronically used drugs can be studied.

PART I

-

Characteristics and Mechanisms of Environmentally and

Biologically Important Adsorbate-Adsorbent Interactions

Adsorption is a surface phenomenon that is characterised by the concentration of a chemical species (adsorbate) from its vapour phase or from solution onto or near the surface or pores of a solid (adsorbent). This surface access occurs in general when the attractive energy of a substance with the solid surface (i.e., the adhesive work) is greater than the cohesive energy of the substance itself and the adsorptive uptake is amplified if the solid material has a high surface area (Manes, 1998:26-68). Amongst the various processes and systems influenced or controlled by adsorption two main areas stands out, (1) adsorption processes occuning in environmental and (2) adsorption occuning inside biological systems.

In the environment the concern for the presence of a wide variety of contaminants calls for the development and assembly of information about their behavioural characteristics so that appropriate strategies can be instituted to either prevent or minimise their adverse impacts on human welfare and natural resources. This information is especially warranted for toxic chemicals that persist in the environment for extended periods of time. Normally when chemicals enter the environment, they are not confined to a specific location but rather are in dynamic motion within a medium or location, or across adjacent media and phases. The propensity for a contaminant to move into and distribute itself between media or phases is determined by its physical and chemical properties and environmental factors and variables. It is therefore important to understand what drives a contaminant from one phase to another and the manner and extent that a contaminant associates with the different components within a local environmental system.

Man early on realised that when intentional or accidental poisoning occurs, in many instances, other materials could be fed to the individual or animal to reduce the effect of the toxicant. Most of these substances, the most commonly used being activated carbon, works by adsorbing the toxic chemical from the gastric tract. In addition the concurrent administration of drugs and medicinal products containing solid adsorbents such as anti-diarrhoea mixtures may result in the adsorbents interfering with the adsorption of such drugs from the gastrointestinal tract. This will reduce the amount of "free" or absorbable drug in solution at

the sites of absorption, which in turn will reduce the rate of drug absorption. Furthermore, if the adsorbed drug is not readily released from the solid adsorbent in order to replace the "free" drug, which has been absorbed, then there will be reduction in the extent of adsorption of the drug.

Part I of this thesis represents a literature review summarising information that deals with issues and concerns related to adsorption processes in biological and environmental systems. The first chapter deals with the principles and processes by which organic contaminants are sorbed to natural and abiotic substances. It focuses on the physicochemical properties and system parameters that affect the uptake by adsorbing materials such as activated carbon, soil, soil substances, medicinal and other natural products. The second chapter highlights examples of environmentally and biopharmaceutically important adsorption processes and the effect that adsorption has on these systems.

Chapter

I

Fundamentals of Adsorption and Adsorption Processes

Introduction

This chapter is a review of certain fundamental aspects of the sorption process. Emphasis is placed on the principles underlying the sorption of compounds to different media and the related absorbent-adsorbate properties. To this end the chapter start with an introduction to thermodynamics and theories of solution and adsorption as it relates to sorption-related thermodynamic processes. This is followed by a distinction between the adsorption of non- ionic compounds to substances by either a partition process (a solution phenomenon) or by an adsorption process (a surface phenomenon), or by both in some situations. In Table 1.1 some principal terms associated with adsorption are defined.

Table 1.1: Definitions; adsorption (Rouquerol et al., 1999:6).

Term Definition Adsorption Adsorbate Adsorptive Adsorbent Chemisorption Physisorption Monolayer capacity Surface Coverage

Enrichment of one or more components in an interfacial layer Substance in the adsorbed state

Absorbable substance in the fluid phase Solid material on which adsorption occurs Adsorption involving chemical bonding Adsorption without chemical bonding

Either chemisorbed material required to occupy all surface sites or Physisorbed amount required to cover surface

Important thermodynamic properties regulating adsorption

In adsorbing systems, one should be keenly interested in the transfer of a chemical one phase to another and in the manner it distributes itself between phases at equilibrium (Chiou, 2002:l). Depending on the material properties of individual phases and on variable environmental factors, such as temperature and humidity, the manner by which a contaminant is retained by individual phases can vary widely. For most organic compounds, the way it is retained by a biotic or abiotic material falls into one or both of two manners. The compound adheres only to the surface of the substrate, or it dissolves into the molecular network of the substrate (Rouquerol et al., 1999:l-25). In other words, mass transfer takes place.

Whether this mass transfer occurs for any compound across phases or the compound at the time is at equilibrium between phases at constant temperature and pressure, no net exchange of mass, is determined by the equality or inequality of its chemical potentials with the various phases (Rouquerol et al., 1999:209). These chemical potentials are the molar Gibbs free energies of the compound in the individual phases. There is a natural tendency of a chemical to come to a state of equilibrium between all contacted phases, where the chemical potential gradients across phase boundaries are zero. The chemical potentials are derived from the first and second laws of thermodynamics. The Gibbs free energy and in particular the thermodynamic properties, enthalpy (heat) and entropy, is also important when one want to distinguish between surface and solution processes (Connors, 2002: 147).

Importance of fundamental solution theory for adsorption

In natural systems, the solubilities of organic compounds in water and other physiological fluids play a crucial role in the behaviour and fate of these compounds. The solubilities not only affect the limit to which a substance can be solubilised by a solvent or a phase, but also dictates the distribution pattern of the substance between two solvents or phases of interest (Chiou, 2002:14). Water is the most important natural solvent, not only because it is abundant, it is also an essential component of all living organisms, and it is a common medium through which various compounds and especially contaminants are transported to other media.

In general it should be recognised that the water solubilities of organic compounds vary much more widely with their structures and compositions than do their corresponding solubilities in an organic-solvent phase (Yalkowsky, 1999:12-15). For liquid substances the solubility in a solvent (or medium) is determined by the degree of solute-solvent compatibility. For solid substances, the solubility is also affected by the energy required to overcome the solid-to- liquid transition (called the melting point effect). These features of compounds related to their solubility immediately suggest that both the potential level of contamination in environmental or biological systems and the distribution pattern may vary widely for the various types of organic compounds (Chiou, 2002:14). Therefore to understand the solubility and partition behaviour of organic compounds in natural systems, it is essential that one capture the essentials of the relevant solution theory. These theories include Raoult's law, Henry's law and the Flory-Huggins theory (Yalkowsky, 1999: 11-1 5).

Raoult (1887; 1430) recognised that the addition of a small amount of solutes to a solvent does not radically change any extensive property of the solvent, because it changes the solvent mole fraction only slightly. On the other hand the properties of a solute may change much more radically as it goes from a pure substance to one in dilute solution. Whereas Raoult's law applies well for a component (generally the solvent) when its mole fractions is close to one, Henry's law applies to components at high dilution. These two laws have their respective advantages in describing solution and partition processes, depending on the system involves. If a substance of interest is either completely miscible with the solvent or has a very high solubility in the solvent, Henry's law is preferred to account for the behaviour of the substance in the dilute range, and Raoult's law is preferred in the high concentration range; this minimises the effort to characterise the system over the whole concentration range (Chiou, 2002: 19).

In contrast two Raoult's and Henry's laws, the Flory-Huggins theory provides a more accurate treatment for systems where the differences in molecular size between the components are considerable, such as for common polymeric and macromolecular substances (Yalkowsky, 1999:ll-15). Generally, small molecules behave more like rigid bodies, while large molecules contain many relatively flexible repeating units that enable them to take on a large number of spatial orientations. The segments can interact relatively freely with each

other and with other molecular species. In this sense, large molecules behave as many independent small molecules and therefore, the mole fraction concept, as adopted by Raoult's law, is not an effective measure of the component activity in a solution. The Flory-Huggins theory offers a more accurate and rigorous treatment of the chemical activity of a component in a macromolecular solution in terms of its volume fraction (Chiou, 2002: 19-2 1).

These laws together with other measured properties of compounds such as the molar heat of solution, cohesive energies and solubility parameters can be used to account for the solubility and partition behaviour of organic compounds with various solvents, natural organic substances, and biological compounds including lipids.

Partition between two separate phases

Using solution theory, we can conveniently express the chemical potential of a compound in solution at a specific temperature and constant pressure in terms of its concentration and its pure-liquid or supercooled-liquid reference chemical potential at that temperature. This is important because to establish the equilibrium partition coefficient of an organic solute between two separable solvent phases, one equates the chemical potential of the solute in one phase with that in the other (Connors, 2002:91). If we designate the phases of interest

as

A and B, the equality in chemical potential of the solute in phase A and B require that the solute activities in the two phases are identical at equilibrium. This equilibrium or partition coefficient of the solute is expressed as the ratio of the solute molar concentrations in the two phases. When dealing with interphase partitioning in environmental and biological systems we are particularly interested in (Chiou, 2002:30-38):

Partition between an organic solvent and water Partition between a macromolecular phase and water Temperature dependence of partition coefficient Concentration dependence of partition coefficient

Fundamentals of the adsorption theory

As stated earlier, adsorption is a surface phenomenon characterised by the concentration of a compound (adsorbate) from its vapour phase or from solution onto or near the surfaces or pores of a solid (adsorbent) (Chiou, 2002:39). This surface excess occurs in general when the attractive energy of a substance with the solid surface (the adhesive work) is greater than the cohesive energy of the substance itself (Manes, 1998:26-68). According to Manes (1998:26- 68) the adsorptive uptake is amplified if the solid material has a high surface area. If the adsorption occurs by London-van der Wads forces of the solid and adsorbate, it is called physical adsorption (physisorption, Table 1.1). If the forces leading to adsorption are related to chemical bonding forces, the adsorption is referred to as chemical adsorption (chemisorption, Table 1.1). However, the distinction between these two processes is not always sharp.

From a thermodynamic point of view, the concentration of a substance from a dilute vapour phase or solution onto a solid surface corresponds to a reduction in freedom of motion of molecules and thereby a loss in system entropy. As such, the adsorption process must be exothermic to the extent that the negative AH is greater in magnitude than the associated negative T AS to maintain a favourable free-energy driving force (Le., for AG to be negative). For a more detailed discussion on the thermodynamics of the adsorption process the reader is referred to Adamson (1967:563), Gregg and Sing (1982:126) and Manes (1998:26-68). When a vapour is adsorbed onto a previously unoccupied solid surface or its pore space, the amount adsorbed is proportional to the solid mass. Adsorption also depends on the temperature (T), the equilibrium partial pressure of the vapour (P), and the nature of the solid and vapour. For a vapour adsorbed on a solid at a fixed temperature, the adsorbed quantity per unit mass of the solid (Q) is then only a function of P. The relation between Q and P at a given temperature is called the adsorption isotherm. For adsorption of solutes from solution similar isotherms are constructed by relating Q with C (the equilibrium concentration) or with the relative concentration, C/Cs, where Cs is the solubility of the solute.

A number of adsorption isotherms have been reported for vapours on a wide variety of solids. Brunauer (1945:153) grouped the isotherms into five principle cases, Types I to V, as shown in Figure 1.1. Type I is characterised by a Langmuir-type adsorption, which shows a monotonic approach to a limiting value that corresponds theoretically to the completion of a surface monolayer. Type I1 is perhaps most common for physical adsorption on relatively open surfaces, where adsorption proceeds progressively from sub-monolayer to multi-layer. This isotherm exhibits a distinct concave-downward trend at lower relative pressure and a sharply rising curve at high relative pressure. The point B at the knee in the curve, Figure 1.1, signifies completion of a monolayer. This type of isotherm forms the basis of the Brunauer- Friendlich-Teller (BET) adsorption model for surface determination of a solid from the assumed monolayer capacity. A Type 111 isotherm represents a relatively weak gas-solid interaction, as exemplified by the adsorption of water and alkanes on nonporous low-polarity solids such as polytetrafluroethylene (Teflon) (Graham, 1965:4387-4391; Whalen, 1968:443- 448, Gregg and Sing, 1982: 126). In this case the adsorbate does not effectively spread over the solid surface. Type IV and V isotherms are characteristic of vapour sorption by capill& condensation into small adsorbent pores, in which the adsorbent reaches an asymptotic value as saturation pressure is approached. Adsorption of organic vapours on activated carbon is typically Type IV and adsorption of water vapour on activated carbon is Type V (Manes, 1998: 26-68).

These results and descriptions of the isotherms shown in Figure 1.1 are based on the adsorption of vapours. The shape of the adsorption isotherms of a solute from solution depends sensitively on the competitive adsorption of the solvent and other components and may deviate greatly from that of its vapour on the solid.

Figure 1.1: The five types of adsorption isotherms according to the classification of Brunauer (1945:153).

Langmuir adsorption kotherm

The most important model of monolyer adsorption came from the work of Langmuir, which was developed between 19 13 and 19 18. Langmuir considered adsorption of an ideal gas onto the idealised surface. The gas was presumed to bind at a series of distinct sites on the surface of the solid, and the adsorption process was treated as a reaction where a gas molecule A, reacts with an empty site, S, to yield and adsorbed complex Aad:

Different expressions can be derived from the Langmuir adsorption isotherm for a number of examples (Masel, 1996:239-240).

Kinetic derivations were made to develop the Langmuir adsorption isotherm for noncompetitive, nondissociative adsorption (Masel, 1996:241):

OA = K~,,,P~

1

+

K~,,P~

where KA,,, defines the ratio of two constants, PA is the partial pressure of A over the surface and OA is the fraction of the surface sites covered with A.

This equation was one of Langmuir's key results. It predicts that adsorption of a gas on a surface follows a Type 1 adsorption isotherm, Figure 1.1. At low pressures the coverage varies linearly with pressure. However, the coverage saturates with increasing pressure. Since Langmuir's time, the equation has been found to fit a wide variety of adsorption systems. Therefore, the Langmuir's adsorption isotherm is a very general result for chemisorption systems (Masel, 1996:241).

However, the equation assumes that there is only one species adsorbing onto the surface. During a reaction there often will be two or more species adsorbing simultaneously. Those species often compete for the same sites. As a result, a different adsorption isotherm is needed for the competitive adsorption of two different gases (Masel, 1996:244).

Consider two species A and B that compete for the same adsorption sites. Assume that all sites are equivalent, each site can hold at most one molecule of A or of B, but not both, and there are no interactions between adsorbate molecules on adjacent sites. The above equation can be derived for a competitive non-dissociative adsorption (Masel, 1996:244):

The one other case of special importance is when a molecule Dz dissociates into two atoms upon adsorption. The Langmuir adsorption isotherm for dissociative adsorption will be derived by assuming that Dz completely dissociates to two molecules of D upon adsorption, the D atoms adsorb onto distinct sites on the surface of the solid and then move around and equilibrate, all sites are equivalent, each site can hold at most one atom of D and there are no interactions between adsorbate molecules on adjacent sites (Masel, 1996:244).

The equation

00 = K ~ , , , , P ~ ~ " ~

is the Langmuir adsorption isotherm for a dissociative adsorption process (Masel, 1996: 245). These four equations were Langmuir's key results. They revolutionised the way people thought about adsorption. Notice that as PA increases, the coverage of A increases. However, the total amount of adsorption saturates at modest pressures. As a result, one often finds that the coverage of a component is independent of the pressure of the component in the gas phase. In contrast, if one is absorbing gas into a liquid, the amount of gas that absorbs into the liquid usually does not saturate as the pressure of A increases. Hence, adsorption of a gas onto a surface is fundamentally different than absorption of the gas into a liquid (Masel, 1996: 245).

Another interesting effect occurs during the co-adsorption of two gases, A and B. Notice that according to the equation when the partial pressure of A increases the coverage of B decreases. The reduction occurs even though A and B do not interact on a surface. This is in contrast to the situation in a liquid where a reduction occurs only when there is a direct or indirect interaction between A and B. Hence, these equations show that absorption into a liquid is fundamentally different that adsorption onto a surface in that absorption and adsorption follow much different rate laws (Masel, 1996: 245).

Freundlich equation

Over the years there have been many attempts to propose modified adsorption isotherms that account for the deviations. The simplest deviations from the Langmuir adsorption come during adsorption on rough inhomogeneous surfaces. On a rough surface, there are multiple sites available for adsorption. The heat of adsorption varies from site to site. Hence major modifications to the Langmuir adsorption isotherm are needed in such systems (Masel, 1996: 246).

The most important multisite adsorption isotherm for rough surfaces in the Freundlich adsorption isotherm (Masel, 1996: 247):

Where a~ and CF are fitting parameters. This equation implies that if one makes a log-log plot of adsorption data, the data will fit a straight line. The Freundlich adsorption isotherm has two parameters, while Langmuir's equations only have one. As a result, the Freundlich equation often fits adsorption data on rough surfaces better than the Langmuir's equations (Masel, 1996: 247).

However, it was found that this equation usually only fits adsorption data taken over a small pressure range and the equation has little predictive value. As a result, this equation is now rarely used (Masel, 1996: 248).

The Freundlich adsorption isotherm is used mainly for rough inhomogeneous surfaces. With single crystals, it has been more common to assume that the adsorbate can adsorb on a finite number of sites, each of which follows a Langmuir adsorption isotherm. Following Langmuir, one then calculates the total coverage of an A, BA by summing over the individual types of sites (Masel, 1996: 248):

=

C;

XiKlquPA

This equation should in principle be quite useful. In fact, years ago, it was used quite regularly even though it had not been rigorously verified experimentally. In recent years lattice gas calculations have largely replaced the multisite model. However, the multi-site model is still used occasionally (Masel, 1996: 248).

Brunauer-Emmet-Teller (BET) adsorption theory

It was over 60 years ago that Brunauer and Emmett made their first attempts to determine the surface area of an iron synthetic ammonia catalyst by means of low-temperature gas adsorption. Their work has attracted an enormous amount of attention - both support and criticism. Indeed, the BET theory is now known to be based on an over-simplified model of multilayer adsorption (Sing, 1998:4). Generally, the derived values of BET-area can be regarded as effective areas unless the material is ultra-microporous. It is advisable to check the validity of the BET-area by using an empirical method of isotherm analysis. In favourable cases, this approach can be used to evaluate the internal and external areas (Sing, 1998:4).

It is assumed that the adsorption is localised. Unit area of the adsorbent surface contains N, equivalent adsorption sites of which N1 are occupied by adsorbate molecules. The N I molecules thus constitute the first layer of adsorbate. If the total number of adsorbate molecules at the surface is N then (N - NI) molecules are in subsequent layers. Each first

layer of adsorbate acts as a potential adsorption site for a second layer molecule, which in turn acts as a site for a third layer molecule and so on, there being no restriction on the total number of molecules in any given stack. It is supposed that molecules in the second and higher layers have the same partition function and energy as in the b u k liquid. The molecules in the first layer will, in general, have a different partition function. It is further assumed that the stacks do not interact energetically (Aveyard and Haydon, 1973: 161).

The BET equation can be given as :

Some of the shortcomings of the BET model are apparent. The assumption of localised adsorption in all the layers is not obviously in accord with the supposition that the film (excluding the first layer) is liquid. Also, the assumption that the stacks of molecules do not interact energetically is unrealistic. It means that molecules in the 'liquid' layer have only two interacting nearest neighbours, whereas in reality a molecule in a bulk liquid would have up to twelve nearest neighbours (Aveyard and Haydon, 1973:164). In spite of these inadequacies the BET theory is very useful in a qualitative sense and isotherms of Type I1 and Type 111 are described. When the constant c > 1, a Type I1 isotherm is predicted. Here, adsorption in the first layer is strong relative to the adsorption in higher layers, so that the first layer is almost completed before higher layers are formed. This accounts for the formation of the 'knee' in the isotherm at low values of p/pO (Aveyard and Haydon, 1973:164). For small c (say 0.1) yields an isotherm of Type 111. The change from Type I1 to Type 111 occurs for a value of c = 2, when the point of inflexion in the isotherm coincides with the origin (Aveyard and Haydon, 1973: 164).

Polanyi adsorption potential theory

Polanyi advanced a thermodynamic theory of gas adsorption. Because it proposes no detailed molecular model, the theory is less open to criticism than is the BET treatment. The basic concept is that there is a force field surrounding a solid, which influences the adsorbate molecules. The forces are long-range and fall off with distance from the surface (Aveyard and Haydon, 1973:166). In this theory the adsorption potential, e, is defined as the work done by adsorption forces in the transference of an adsorbate molecule from the bulk gas to a point in the surface phase. Since the force of attraction decreases with distance from the surface, E

also decreases, having a maximum value of €0 at the solid surface. It is possible to construct planes of equipotential around a solid, Figure 1.2 (Aveyard and Haydon, 1973: 166).

Figure 1.2: Rough schematic model for a region of the porous carbon surface @ore) showing the equipotential surfaces corresponding to successively lower values of the adsorption potential with increasing pore size. A vapour liquefies wherever the adsorption potential required to concentrate it to saturation is equalled or exceeded (Chiou, 2002:46).

Consider the simple case of the adsorption of an ideal gas or vapour. Assume that the temperature is well below the critical temperature, so that the adsorbed film is liquid, and that the liquid is incompressible. Then &i is the work required to compress the ideal gas, at

constant temperature, from the pressure, p,, of the gas to the vapour pressure, of the liquid at that temperature. Thus, per mole,

The compression is accomplished by the force field of the solid. The work of creating the interface between the liquid film and the gas is not here taken into account hut this does not lead to serious errors. The volume cpi is given by

where m is the mass adsorbed at a pressure p, and pl is the density of the liquid adsorbate at the temperature of the adsorption (Aveyard and Haydon, 1973:167).

Surface properties and areas of solids

Except for rare cases where the microscopic structure of a solid surface is nearly uniform, the surfaces of most solids are heterogeneous, with the result that adsorption energies are variable (Chiou, 2002:40). Adsorption sites are taken up sequentially, starting from the highest energy sites to the lowest energy sites with increasing partial pressure or solute concentration. Thus, the net or differential molar heat of adsorption decreases with increasing adsorption and vanishes when the vapour pressure or solute concentration reaches saturation. Therefore, adsorption isotherms are typically non-linear because of the energetic heterogeneity and the limited active sites or surfaces of solids. Furthermore, since a given site or surface of a solid cannot be shared by two or more different kinds of adsorbates, the adsorption process is competitive when compared to a partition process. In Table 1.2, some of the principles terms and properties associated with powders and porous solids are defined.

Table 1.1: Definitions; powders and porous solids (Rouquerol et al., 1999: 7-8).

Term Definition

Powders

Powder

Fine powder Aggregate

Dry material composed of discrete particles with a maximum dimension less than about 1 mm.

Powder with particle size below about 1 pm. Loose, unconsolidated assembly of particles.

Agglomerate Compact Acicular Surface area

Specific surface area

External surface

Roughness factor

Divided solid

Rigid, consolidated assembly of particles. Agglomerate formed by compression of powder. Needle-shaped.

Extent of available surface as determined by a given method under stated conditions.

Surface area of unit mass of powder,

as

determined under stated conditions.

Area of external surface of particles, t&ng into account roughness, but not porosity.

Ration of external surface area to area of smoothed surface around particles.

Solid made up of more or less independent particles, which may be in the form of a powder, aggregate or agglomerate.

Porous solids

Porous solid Open pore Closed pore Void Micropore Mesopore Macropore

Solid with cavities or channels that are deeper than they are wide. Cavity or channel with access to the surface.

Cavity not connected to the surface. Space between particles.

Pore with internal width less than 2 nm.

Pore with internal width between 2 nm to 50 nm. Pore with internal width greater than 50 nm.

Pore size Pore width

Pore volume Volume of pores determined by stated method.

Porosity Ratio of total pore volume to apparent volume of particles or powder.

Internal surface area Area of pore walls.

True density Density of solid excluding pores and voids.

Apparent density Density of material including closed and inaccessible pores, as determined by stated method.

The surface area or porosity of solids is usually the principal factor affecting the amount of vapour adsorption; therefore, a powerful adsorbent must have a large surface area. For highly porous solids, the term internal surface is frequently used to refer to the surface associated with the walls of the pores that have narrow openings, which extent inward from the granule surface to the interior of the granule (Gregg and Sing, 1982:126). On the other hand the term external surface is used to refer to the surface from all prominences and those cracks that are wider than they are deep. It is understand that the internal surface is restricted to open-ended pores and does not apply to sealed-off pores. Although, these two kinds of surfaces are operational in their definitions, it is understood that the internal surface is nevertheless external to the material and accessible to adsorbents (Chiou, 2002:49). Thus, as long as the adsorbate does not penetrate the field of force that exists between the atoms, ions, or molecules inside the solid, it is considered to be on the external surface, despite the fact that it may adsorb on the solid's internal surface (Brunauer, 1945:153). For highly micro-porous solid such as activated carbon, one may then say that the solid has a very high surface area, as determined by the BET method, because it has a high internal surface.

Isosteric heat of adsorption

Adsorbent surfaces are commonly energetically heterogeneous therefore the exothermic heat of adsorption of a vapour (or a solute) usually varies with the amount adsorbed (Chiou, 200250). To account for the variation in the adsorption heat, the isosteric heats of adsorption at some fixed adsorbate loadings are determined from the equilibrium vapour pressures (or solute concentrations) of the isotherms at different temperatures, Figure 1.3, with the aid of the Clausius-Clapeyron equation (Connors, 2002:49). Isosteric-heat data describe how the molar heat of adsorption of a vapour or a solute varies with the amount adsorbed by a solid. Because the sorption of organic compounds to many natural solids may be dictated by processes other than adsorption (e.g., by a partition interaction), the isosteric plot of the isotherms provides useful heat data for the undergoing process.

For example, in a typical partition process of an organic solute from water to a partially miscible organic phase, the isotherm is usually highly linear over a wide concentration range, and the molar isosteric heat of sorption is largely constant, independent of solute concentrations (Chiou, 200252). This unique characteristic enables the distinction between a partition and an adsorption process such as for ordinary soils which act as a dual adsorbent in the uptake of organic compounds, because either adsorption on soil minerals or partition into soil organic matter may dominate the soil uptake. The detected isostezic heat for the system helps to pinpoint the dominant mechanism.

Adsorbent partition and concentration

The partition of organic compounds in partially miscible solvent-water systems has been used in chemistry as the basis for extraction solutes from water. About a century ago researchers started to investigate the use of partition coefficients in biochemical systems (Meyer, 1899:109-118; Overton, 1901:85). They showed that the relative narcotic activities of drugs correlated well with their oil-water partition coefficients. After these first reports the usefulness of the partition coefficient as a system for assessing the biochemical activity of organic compounds or drugs has been greatly extended. Leo and Hansch (1971:1539-1544) reviewed the partition characteristics of organic compounds in a variety of solvent-water systems and for practical reasons considered the octanol-water system the most appropriate

reference for assessing the relative lipophilicity or organic solutes. They showed for example, that the partition coefficients of organic solutes between protein and water could be correlated successfully with their octanol-water partition coefficients, thus providing an assessment of the binding of small organic molecules with biological macromolecules.

The utility of partition coefficients to estimate the distribution of organic contaminants in environmental systems has also become increasingly evident. This is because the potential of an organic contaminant to concentrate from water into aquatic organisms may be correlated with its octanol-water partition coefficient (Oliver and Nimii, 1983:287-291; Chiou, 1985:57- 62). Similar empirical correlations with the octanol-water coefficients were also found for soil(sediment)-water distribution coefficients for certain groups of organic compounds (Chiou, 2002:53; Briggs, 1981 : 1050-1059). Although contaminant distribution between water and natural organic substance are usually more complicated than simple partition, these studies showed that the driving force for contaminant distribution in organic substrates is conceptually analogous to the solvent-water partition process (Chiou, 2002:54). These kinds of correlations are important because a major concern for environmental contamination is the extent (bio-concentration factor, BCF) to which pollutants concentrate from water into aquatic organisms such as fish. The BCF is the ratio of the pollutant concentration in fish to that in water.

Conclusion

In adsorbing systems, one should be keenly interested in the transfer of one chemical phase to another and in the manner it distributes itself between phases in equilibrium. In natural systems, the solubilities of organic compounds in water and other physiological fluids play a crucial role in the behaviour and fate of these compounds. The solubilities not only affect the limit to which a substance can he solubilised by a solvent or a phase, but also dictates the distribution pattern of the substance between two solvents or phases of interest. Therefore to understand the solubility and partition behaviour of organic compounds in natural systems, it is essential that one capture the essentials of the relevant solution theory. These theories include Raoult's law, Henry's law and the Flory-Huggins theory. These laws together with other measured properties of compounds such as the molar heat of solution, cohesive energies

and solubility parameters can be used to account for the solubility and partition behaviour of organic compounds with various solvents, natural organic substances, and biological compounds including lipids.

Adsorption is a surface phenomenon characterised by the concentration of a compound from its vapour phase or from solution onto or near the surfaces or pores of a solid. This surface excess occurs in general when the attractive energy of a substance with the solid surface is greater than the cohesive energy of the substance itself. There has been a number of adsorption isotherms reported for vapours on a wide variety of solids. These isotherms have been grouped into five principle cases, Type I to V. Type I is characterised by the Langmuir- type adsorption, Type I1 is characterised by the Bmnauer-Emmet-Teller (BET) adsorption model. A Type 111 isotherm represents a relatively weak gas-solid interaction, while Type IV and V are characteristic of vapour sorption by capillary condensation into small adsorbent pores.

Surface properties and areas of solids also play an important role in adsorption. Most surfaces of solids are heterogeneous, with the result that adsorption energies are variable. Adsorption sites are taken up sequentially, starting from the highest energy sites to the lowest energy sites with increasing partial pressure or solute concentration. The adsorption isotherms are typically non-linear because of the energetic heterogeneity and the limited active sites or surfaces of solids. The surface area or porosity of solids is usually the principal factor affecting the amount of vapour adsorption. A powerful adsorbent must have a large surface area, which could consist of only and external surface of an internal surface for highly porous solids.

Chapter 2

Adsorption of Organic Substances in Environmental and Biological

Systems

Introduction

If we define adsorption as the process of accumulation at an interphase then adsorption in environmental and biological systems is essentially a surface effect and should be distinguished from absorption, which implies the penetration of one component throughout the body of a second. In the environment the dynamic movement of contaminants within a phase or between phases increases the possibility of it being adsorb and thereby persist for extended periods. In biological systems adsorbents generally are non-specific and will adsorb nutrients, drugs and enzymes. In both environmental and biological systems several factors affect the extent and ease of adsorption.

As mentioned in Chapter 1, solubility is an important factor affecting adsorption. In general the extent of adsorption of a solute is inversely proportional to its solubility in the solvent from which adsorption occurs. In order for adsorption to occur, solute-solvent bonds must first be broken. The greater the solubility, the stronger are these bonds and hence the smaller the extent of adsorption (Florence and Attwood, 1998:194).

Other factors affecting adsorption are pH, the nature of the adsorbent and the temperature. The most important affect pH has on adsorption is the effect on the ionisation of the adsorbate drug molecule. Adsorption increases as the ionisation of the drug is suppressed. Thus, adsorption reaches a maximum when the drug is completely unionised. For amphoteric compound, adsorption is at a maximum at the isoelectric point; that is, when the compound bears a net charge of zero. In general, pH and solubility effects act in concert, since the unionised form of most drugs in aqueous solution has a low solubility (Florence and Attwood, 1998:195).

The physicochemical nature of the adsorbent can have profound effects on the rate and capacity of adsorption. The last factor, which generally can influence the adsorption, is the temperature by which the process occurs. Since adsorption is generally an exothermic process, an increase in temperature normally leads to a decrease in the amount adsorbed (Florence and Attwood, 1998: 197).

Adsorption to soil and other natural adsorbents

Clays are the main components of the mineral fraction of soils. They are effective natural adsorbents due to their particle size (lower than 2 pm), lamellar structure and negatively charged surfaces, which make them good adsorbents by ion exchange (Tsai et aL, 2002:189). For example, bentonite clays are widely used in various industrial products and processes such as paints, coatings, ceramics, pesticides, pharmaceuticals, cosmetics, cement and drilling fluids to modify the rheology and control the stability of the systems. In addition, clay suspensions are powerful adsorbents and are much cheaper than common adsorbents, like activated carbon, to remove polymers from industrial wastewaters (0ztekin et al., 2002:73). Because of the expensiveness of most adsorbents, Viraraghavan and Alfaro (1998:59) examined the effectiveness of less expensive natural adsorbents such as peat, fly ash and bentonite in removing phenol form wastewater. Clay minerals such as kaolinite, illite and montmorillonite can also be used as adsorbates (Pernyeszi et al., 1998:373). Clays can also be used to adsorb radioactive waste (Nagy et al., 1999:245).

In most natural systems adsorption is affected by complex forming agents, e.g. humic acids of the soil, vegetable acids and complex forming agents applied in fertilisers. Complex forming agents can desorb metal ions from the soil and introduce them into the food chain. On the other hand, the soils polluted with toxic cations or radioactive isotopes can be decontaminated by complex forming agents (Nagy et al., 1999:245).

To demonstrate the adsorption of toxins, Lenoble et al. (2002:52) studied arsenic adsorption on simple materials such as goethite and amorphous iron hydroxide, and more complex matrices such as clay pillared with titanium (IV), iron (111) and aluminium (111). Arsenate elimination was favoured at acidic pH, whereas optimal arsenite elimination was obtained at

pH between 4 and 9. For pH values above 10, the pillared clays were damaged and elimination deceased. Amorphous iron hydroxide had the highest adsorption capacities both towards arsenate and arsenite. Desorption experiments from the various matrices were also carried out and iron- and titanium-pillared clays showed a desorption capacity above 95% and around 40% respectively, but no desorption rate could be obtained for iron hydroxide as they were damaged during the process.

Pesticides are used daily is nature and contamination are a big risk. To determine whether adsorption is a factor, Tsai et al. (2003:29) studied the adsorption of paraquat onto different particle sized activated clay powders. From their experimental results, the adsorption process could be described as a pseudo-second order process with a Freundlich adsorption isotherm and the rate constant (k) of paraquat adsorption decreased with increasing particle size.

Sotption from aqueous solutions, o*ganic solvents and the vapourphase

Water pollution is one of the most important environmental problems in the world. Faecal pollution of water, especially drinking water, has frequently caused waterborne diseases. In general, waterborne diseases have been well controlled, especially in developed countries. However, waterborne toxic chemicals pose a great threat to the safety of water supplies in developed and developing countries alike. There are many sources of toxic chemicals in the environment, such as badly managed landfills, industrial pollution and pesticide runoff. However, industrial wastewater is an important point source of water pollution. Microbial degradation, chemical oxidation, photolysis and adsorption are used for the treatment of wastewater. Activated carbons are made from different plants, animal residues and bituminous coal (Daifullh et al., 2003: 1723).

Because of the high cost of activated carbon, Rengaraj et al. (2002543) searched for new and low cost agricultural wastes as source material for activated carbon. For this purpose, attempts were made to produce activated carbon from palm seed coat by the dolomite process. They found that adsorption capacity of phenol onto palm seed coat activated carbon was 18.3 mglg for the particle size 20

-

50 mesh. This capacity is superior to that of many commercial activated carbons. The adsorption of phenol followed first order reversible kinetics with film diffusion being the essential rate controlling step.

To improve the adsorption properties of activated carbon, Poleart et al. (2002:1585) also studied the treatment of wastewater to remove phenol by

a

two-step adsorption-oxidation process on activated carbon. This process is based on the use of activated carbon as adsorbent in the first step and as oxidation catalyst in the second step, in a single bifunctional reactor. In the fust step, adsorption of the pollutant occurs and in the second step, the activated carbon is used as a catalyst for oxidation reactions. Although this study shows good potential for future treatment of wastewater, the economical aspects has not been studied in detail.

In another study Frimmel et al. (2002:731) compared the adsorption behaviour of organic substances from industrial wastewater onto activated carbon or an organic polymer resin (Lewatit EP 63). In equilibrium experiments the polymer resin showed a lower adsorption capacity than activated carbon for wastewater constituents from different origin. However, the polymer was more selective which can be advantageous for technical applications in which organic substances have to be removed selectively. The adsorption on activated carbon showed no selectivity.

Heavy metal pollution is serious ands may come from various industrial sources such as electroplating, metal f ~ s h i n g , textile, storage batteries, lead smelting, mining, plating, ceramic and glass industries. The method for removal of many heavy metals include precipitation, oxidation, reduction, ion exchange, filtration, electrochemical treatment, membrane technologies, reverse osmosis and solvent extraction. Adsorption is a well established technique for heavy metal removal and activated carbon is the most widely used adsorbent. However, the use of activated carbon can be expensive and there has been considerable interest in the use of other adsorbent materials, particularly biosorbents. This technique is now recognised as an alternative method for the treatment of wastewaters containing heavy metals (Keskinkan et al., 2003:179). Daifullah et al. (2003:1723) prepared two adsorbents from rice husk, a by-product of rice milling industry. Rice husk was chosen due to its granular structure, insolubility in water, chemical stability, high mechanical strength and its availability at almost no cost. When the adsorption capacity of the two sorbents were compared, it was found that six heavy metals found in Egyptian wastewater, was almost 100% adsorbed and the final concentration of any metal after treatment was insignificant. The two sorbents were capable of removing all the metals from thee complex matrix under.

Adsorbents are also frequently used to remove pesticides from the environment. For virtually all pesticides, granular activated carbon filter has been considered as the best available

technology. However, the cationic pesticide, paraquat, adsorbed strongly to clay minerals, and somewhat less on activated carbon due to its highly polar nature for an expanding lattice clay, like montmorillonite (Tsai et aL, 2003:29).

Influence of sorption on adsorbate activiiy

When an adsorbent is adsorbed onto a solid surface, the resultant effect on the character of that surface depend largely on the dominant mechanisms of adsorption (Meyers, 1999:210- 211). For example, for a charged surface, if adsorption is a result of ion exchange, the electrical nature of the surface will not be altered significantly. However, if ion pairing becomes important, the potential at the interface will decrease until it is completely neutralised. If the adsorbed molecules are amphiphilic such as a surfactant, Figure 2.1, adsorption by ion exchange or ion pairing results in the orientation of the molecules with hydrophobic groups towards the aqueous phase. The surface then becomes hydrophobic and less wettable. If adsorption continue by dispersion force interactions, Figure 2.1, it can reverse the change on the surface, because the hydrophilic groups are now orientated towards the aqueous phase.

Figure 2.1: The interaction of an ionic surfactant with a surface of opposite charge will lead to charge neutralisation (a) followed, in may case, by charge reversal (b). Counterions are omitted for clarity (Meyers, 1999:210).

Contaminant uptake by plants from soil and water

It has been long known that aquatic plants, both living and dead, are heavy metal accumulators and therefore, the use of aquatic plants for the removal of heavy metals from wastewater gained high interest. Some freshwater macrophytes including Potamogeton lycens, Salvivia herzogoi, Eichhornia crassipes, Myriophyllum brasillensis, Myriophyllum spicatum, Cabomba sp., Ceratophyllum demersum have been investigated for the removal of heavy metals (Keskinkan et al., 2003: 179). These researchers found that M. spicatum could be effective as a biosorbent for the removal of zinc, lead and copper. Batch adsorption studies showed that, based on the Langmuir coefficients, the maximum adsorption capacity was 15.59 mg/g for zinc, 46.49 mg/g for lead and 10.37 mglg for copper. The overall adsorption rate showed that zinc, copper, lead adsorption in the M. spicatum system was best described by the pseudo second order model (Keskinkan et al., 2003: 179).

Pharmaceutically important adsorption at the solid-liquid interface

The phenomenon of adsorption from solution finds practical application of pharmaceutical interest in chromatographic techniques and in the removal of unwanted materials. In addition adsorption gives rise to certain formulation problems. For example materials such as activated charcoal can be given in cases of orally taken poisons to adsorb the toxic materials. In addition, adsorbents may be used in haemodialysis to remove the products of dialysis from the dialysing solution and hence allowing the solution to be recycled. Adsorption may give problems in formulations where drugs or other materials such as preservatives are adsorbed by containers, thus reducing the effective concentration. For example vapours formed by the volatile glyceryl trinitrate, may be sorbed by the container leading to further volatilisation and loss of potency. The adsorption of insulin on to intravenous administration sets has also been reported. Certain preservatives such as phenylmercuric acetate used in eye drops are also adsorbed onto polyethylene containers. Adsorbents may also be used in haemodialysis to remove the products of dialysis from the dialysing solution and hence allowing the solution to be recycled (Florence and Atwood, 1988: 198).